Stephanie is 4 times as old as William and is also 15 years older than William. How old is William?
We can use the given information to write down two equations that describe the ages of Stephanie and William. Let Stephanie's current age be $s$ and William's current age be $w$ $s = 4w$ $s = w + 15$ Now we have two independent equations, and we can solve for our two unknowns. Since we are looking for $w$ , and both of our equations have $s$ alone on one side, this is a convenient time to use elimination. Subtracting the second equation from the first equation, we get: $0 =$ $4w$ $-$ $ (w + 15)$ which combines the information about $w$ from both of our original equations. Solving for $w$ , we get: $3 w = 15$ $w = 5$.